The present invention relates to a multiple-circuit hydraulic brake system, in particular for automotive vehicles, comprising a brake-actuating device for the actuation of wheel brake cylinders which contains three working chambers adapted to be pressurized, a first brake circuit connected to the first working chamber leading to wheel brake cylinders of the rear axle, and second and third brake circuits to connect wheel brake cylinders of the front axle to the second and the third working chamber of the brake-actuating device.
Such a brake system is known from German Pat. DE-OS No. 2,213,944. The known brake system comprises as a brake-actuating device a so-called three-chamber master cylinder, wherein a first piston is displaceable by means of a brake pedal and two other pistons are guided in a cylinder bore hydraulically coupled to the first piston. Thus, the brake-actuating device includes three working chambers, with a first brake circuit leading from a first working chamber to the wheel brake cylinders of the rear axle through the intermediary of a brake-force control valve.
Two wheel brake cylinders are provided at each wheel of the front axle. The front-axle wheel brake cylinders are connected to the second and the third working chamber of the brake-actuating device, respectively, such that simultaneously one wheel brake cylinder of each front wheel is in communication with one working chamber of the brake-actuating device.
Such an arrangement safeguards the occurrence of an even braking of the vehicle in the event of failure of a circuit. If, for instance, the first brake circuit pertaining to the rear axle fails, the dimensioning of the brake system ensures that the vehicle comes to a standstill by actuation of the wheel brake cylinders of the front axle alone, while a yawing moment is avoided. It is furthermore guaranteed by the brake-circuit split-up chosen that the vehicle can be braked with minimum deceleration while having the same brake effect on both vehicle sides.
A disadvantage of the prior art system described is that two wheel brake cylinders are required at each wheel of the front axle which makes the system as a whole more expensive. In addition, the length of the brake lines increases in comparison with conventional systems having a so-called Y-split-up, in which a first brake circuit serves to actuate the wheel brake cylinders of the rear axle while through second and third brake circuits exclusively one wheel brake cylinder at one front wheel, respectively, is actuated. Furthermore, it may happen in the event of a defect or of failure of the brake-force control valve at the rear axle or under extreme load conditions of the vehicle that the wheels of the rear axle will be overbraked, which would result in the vehicle becoming uncontrollable at once.